[mono] Add some WASM AOT documention. (#55498)

docs/design/mono/wasm-aot.md

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+# WebAssembly AOT code generation
+
+## Basic operation
+
+The LLVM backend of the Mono JIT is used to generate an llvm .bc file for each assembly, then the .bc files are
+compiled to webassembly using emscripten, then the resulting wasm files are linked into the final app. The 'bitcode'/'llvmonly'
+variant of the LLVM backend is used since webassembly doesn't support inline assembly etc.
+
+## GC Support
+
+On wasm, the execution stack is not stored in linear memory, so its not possible to scan it for GC references. However, there
+is an additional C stack which stores variables whose addresses are taken. Variables which hold GC references are marked as
+'volatile' in the llvm backend, forcing llvm to spill those to the C stack so they can be scanned.
+
+## Interpreter support
+
+Its possible for AOTed and interpreted code to interop, this is called mixed mode.
+For the AOT -> interpreter case, every call from AOTed code which might end up in the interpreter is
+emitted as an indirect call. When the callee is not found, a wrapper function is used which
+packages up the arguments into an array and passes control to the interpreter.
+For the interpreter -> AOT case, and similar wrapper function is used which receives the
+arguments and a return value pointer from the interpreter in an array, and calls the
+AOTed code. There is usually one aot->interp and interp->aot wrapper for each signature, with
+some sharing. These wrappers are generated by the AOT compiler when the 'interp' aot option
+is used.
+
+## Null checks
+
+Since wasm has no signal support, we generate explicit null checks.
+
+## Issues
+
+The generated code is in general much bigger than the code generated on ios etc. Some of the
+current issues are described below.
+
+### Function pointers
+
+The runtime needs to be able to do a IL method -> wasm function lookup. To do this, every
+AOT image includes a table mapping from a method index to wasm functions. This means that
+every generated AOT method has its address taken, which severely limits the interprocedural
+optimizations that LLVM can do, since it cannot determine the set of callers for a function.
+This means that it cannot remove functions corresponding to unused IL methods, cannot
+specialize functions for constant/nonnull arguments, etc.
+The dotnet linker includes some support for adding a [DisablePrivateReflection] attribute to
+methods which cannot be called using reflection, and the AOT compiler could use this
+to avoid generating function pointers for methods which are not called from outside the
+AOT image. This is not enabled right now because the linker support is not complete.
+
+### Null checks
+
+The explicit null checking code adds a lot of size overhead since null checks are very common.
+
+### Virtual calls
+
+Vtable slots are lazily initialized on the first call, i.e. every virtual call looks like this:
+```C
+vt_entry = vtable [slot];
+if (vt_entry == null)
+	vt_entry = init_vt_entry ();
+```
+
+### GC overhead
+
+Since GC variables are marked as volatile and stored on the C stack, they are loaded/stored on every access,
+even if there is no GC safe point between the accesses. Instead, they should only be loaded/stored around
+GC safe points.